Orientation estimation apparatus, orientation estimation method, and computer-readable recording medium storing orientation estimation computer program

ABSTRACT

An orientation estimation apparatus estimates a position and an orientation of an image capturing unit based on positions of a marker on the image and in the real space when the maker is detected; and based on positions of feature points on the captured image and in the real space, and initial values of the position and the orientation of the unit when the maker is not detected. Further, the apparatus calculates a failure index from a first image acquired while the position and the orientation are estimated by the position of marker, where the failure index indicates a possibility that the detection of the marker is failed. When the failure index satisfies a shift-preparation-start reference, the initial values are set from the position and the orientation estimated by the position of the marker on the image having been acquired in a predetermined period before the first image.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on and claims the benefit of priority of theprior Japanese Patent Application No. 2014-034439, filed on Feb. 25,2014, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to an orientationestimation apparatus, an orientation estimation method, and acomputer-readable medium storing orientation estimation computer programthat estimate, based on an image taken by a camera, for example, aposition and an orientation of the camera.

BACKGROUND

In recent years, to support user works, a study relating to a so-calledAugmented Reality (AR) in which various information is superimposed anddisplayed on an image taken by a camera has been conducted (for example,see Ishii et al., “Proposal and Evaluation of Decommissioning SupportMethod of Nuclear Power Plants using Augmented Reality”, Transactions ofthe Virtual Reality Society of Japan, 13(2), pp. 289-300, June 2008).

In a system that implements the AR, in order that information besuperimposed on a suitable position on an image taken by a camera, it isrequested that a virtual space is accurately aligned to a real space.For that purpose, it is requested that a position and an orientation ofthe camera in the real space may be accurately grasped. Therefore,proposed is a technique to estimate a position and an orientation of thecamera from a position and a shape of a known marker installed inadvance on the image (see, for example, Kato et al., “An AugmentedReality System and its Calibration based on Marker Tracking”,Transactions of the Virtual Reality Society of Japan, 4(4), pp. 607-616,December 1999).

However, it is difficult to accurately detect a marker on the image insome cases depending on photographed conditions. Moreover, a markercomes off from a photographed range of the camera, and as a result, themarker is not picked up on the image in some cases. In such a case, itis difficult to estimate an orientation of the camera based on themarker. Therefore, proposed is a technique to estimate a position and anorientation of the camera based on multiple feature points extractedfrom a photographed image (see, for example, Japanese Laid-open PatentPublication No. 2005-326274).

SUMMARY

According to an aspect of the invention, an orientation estimationapparatus includes an image capturing unit that creates an image of aphotographed range at a predetermined photographing cycle; a storageunit that stores therein a position of a marker installed in advance ina real space and positions of a plurality of feature points in asurrounding of the marker in the real space; a marker detection unitthat detects the marker from the image; a marker-based orientationestimation unit that estimates, based on a position of the marker on theimage and the position of the marker in the real space, a position andan orientation of the image capturing unit; a feature point detectionunit that detects the plurality of feature points from the image; afeature point-based orientation estimation unit that estimates theposition and the orientation of the image capturing unit based onpositions of the plurality of feature points on the image, the positionsof the plurality of feature points in the real space, and initial valuesof the position and the orientation of the image capturing unit; a shiftdetermination unit that causes, when the marker is not detected from theimage, the feature point detection unit to detect the plurality offeature points with respect to the image and a subsequent image to beacquired after the image, and the feature point-based orientationestimation unit to estimate the position and the orientation of theimage capturing unit; a shift preparation start determination unit thatcalculates, from a first image acquired while the marker-basedorientation estimation unit estimates the position and the orientationof the image capturing unit, at least one failure index indicating apossibility that the detection of the marker is failed, and determineswhether or not the at least one failure index satisfies a shiftpreparation start reference; and an initial orientation setting unitthat sets, when the at least one failure index satisfies the shiftpreparation start reference, the initial values of the position and theorientation of the image capturing unit from the position and theorientation of the image capturing unit estimated based on the positionof the marker on the image having been acquired in a predeterminedperiod before the first image.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a hardware configuration diagram of a mobile terminal that isone embodiment of an orientation estimation apparatus;

FIG. 2 is a view illustrating an example a marker;

FIG. 3 is a view illustrating an example of a three-dimensional map inwhich three-dimensional coordinates of the marker, and three-dimensionalcoordinates and the feature amount of each natural feature point arestored;

FIG. 4 is a functional block diagram of a control unit;

FIG. 5 is a view illustrating an example of a list in which a positionand an orientation of an image capturing unit 3 for every imageacquisition time;

FIG. 6A to FIG. 6D are views each illustrating a case where there is apossibility that a marker may not be detected from an image;

FIG. 7 is a conceptual diagram of initial value prediction in naturalfeature point-based orientation estimation processing;

FIG. 8 is a conceptual diagram of the natural feature point-basedorientation estimation processing; and

FIG. 9 is an operation flowchart of the orientation estimationprocessing.

DESCRIPTION OF EMBODIMENTS

To estimate a position and an orientation of a camera based on multiplefeature points, more calculations than those when a position and anorientation of a camera is estimated based on a marker is made becausecalculation to associate a point on a three-dimensional space with apoint on a two-dimensional image is repeated. The more the amount ofcalculation is, the more the power consumption of a hardware thatexecutes the calculation becomes. Meanwhile, when the AR is mounted on adevice with the reduced capacity of a power supply, such as a mobileterminal with a camera, the lesser amount of calculation is preferable.

Consequently, it is desired to provide an orientation estimationapparatus that may restrict, even when a marker may not be detected froman image taken by an image capturing unit, lowering of the estimationaccuracy of a position and an orientation of the image capturing unit,and may restrict the amount of calculation.

Hereinafter, an orientation estimation apparatus will be described withreference to the drawings. Every time an image capturing unit takes animage of a surrounding thereof at a predetermined photographing cycleand creates an image thereof, the orientation estimation apparatusestimates, based on the image, a position and an orientation of theimage capturing unit when taking the image. At that time, when apre-installed marker of which shape and install position are known ispicked up on the image and the marker may be detected from the image,the orientation estimation apparatus estimates a position and anorientation of the image capturing unit based on the position of themaker on the image to restrict the amount of calculation. In contrast,when the marker may not be detected from the image, the orientationestimation apparatus detects, from the image, multiple feature points(hereinafter, referred to as natural feature points) that are present inthe surrounding of the marker and are picked up on the image, andestimates a position and an orientation of the image capturing unitbased on the feature points.

Moreover, when determining that the marker may not highly likely bedetected from the image, the orientation estimation apparatus applies aprediction filter to a position and an orientation of the imagecapturing unit based on the marker on a previously acquired image tostart estimation of a position and an orientation of the image capturingunit. Further, when the orientation estimation apparatus may not detectthe marker from the image and switches the orientation estimation toorientation estimation based on natural feature points, the orientationestimation apparatus uses the position and the orientation of the imagecapturing unit estimated by the prediction filter as initial values toexecute estimation of a position and an orientation based on naturalfeature points. With this, the orientation estimation apparatusrestricts lowering of the estimation accuracy when estimating a positionand an orientation of the image capturing unit based on natural featurepoints.

FIG. 1 is a hardware configuration diagram of a mobile terminal that isone embodiment of an orientation estimation apparatus. A mobile terminal1 includes a display unit 2, an image capturing unit 3, a storage mediumaccess device 4, a storage unit 5, and a control unit 6. The displayunit 2, the image capturing unit 3, the storage medium access device 4,the storage unit 5, and the control unit 6 are disposed in a housing 7.Note that, the mobile terminal 1 is, for example, a mobile telephoneset, a mobile information terminal, or a tablet type computer. Inaddition, the mobile terminal 1 may include a communication interfacecircuit (not illustrated) for connecting the mobile terminal 1 to otherdevices. It is noted that FIG. 1 is a diagram for explaining componentsincluded in the mobile terminal 1, but is not a diagram representingactual arrangements of the respective components of the mobile terminal1.

The mobile terminal 1 estimates a position and an orientation of theimage capturing unit 3, based on a marker or natural feature points thatare picked up on an image obtained such that the image capturing unit 3takes images of the surrounding at every predetermined photographingcycle. Further, the mobile terminal 1 may use estimation results of theposition and the orientation to superimpose various information on theimage, thereby providing AR to a user.

The display unit 2 includes, for example, a liquid crystal display or anorganic electroluminescent display, and is disposed in such a mannerthat a display screen of the display unit 2 is directed to the user whois opposed to a front surface of the housing 7. Further, the displayunit 2 displays various information such as the image created by theimage capturing unit 3, to the user. Moreover, the display unit 2 maypreferably include a touch panel display. In this case, the display unit2 displays, for example, various icons or operation buttons in responseto a control signal from the control unit 6. Further, the user touches aposition of the icon or the operation button that is displayed, thedisplay unit 2 creates an operation signal in accordance with theposition, and outputs the operation signal to the control unit 6.

The image capturing unit 3 includes, for example, an image sensor thatincludes solid image pickup elements disposed in a two-dimensional arrayform, and an imaging optical system that forms an image of a subject onthe image sensor.

The image capturing unit 3 takes the images of a surrounding of themobile terminal 1 at every predetermined photographing cycle to createan image at this photographing cycle. Further, an image to be createdmay preferably be a color image represented by an RGB colorimetricsystem, or may preferably be a gray image. Further, every time the imagecapturing unit 3 creates an image, the image capturing unit 3 outputsthe created image to the control unit 6. Further, the photographingcycle is, for example, 33 msec.

The storage medium access device 4 is, for example, a device thataccesses a storage medium 8 such as a semiconductor memory card. Thestorage medium access device 4 reads in, for example, a computer programthat is stored in the storage medium 8 and is to be executed on thecontrol unit 6, and passes the computer program to the control unit 6.Moreover, as is described later, when the control unit 6 executes acomputer program that implements a function of an orientation estimationapparatus, the storage medium access device 4 may preferably read in anorientation estimation computer program from the storage medium 8, andpass the orientation estimation computer program to the control unit 6.

The storage unit 5 includes, for example, a read/writable nonvolatilesemiconductor memory and a read/writable volatile semiconductor memory.Further, the storage unit 5 stores therein various kinds of applicationprograms that are executed on the control unit 6 and various kinds ofdata. Moreover, the storage unit 5 stores therein various kinds of datathat are used for orientation estimation processing. For example, thestorage unit 5 stores therein information indicating a position and anorientation of the image capturing unit 3 that is estimated from imagesacquired during the latest certain period (for example, a periodcorresponding to several frames to several tens of frames).

In addition, the storage unit 5 stores therein a three-dimensional mapin which three-dimensional coordinates of a marker, three-dimensionalcoordinates of each of multiple natural feature points, and a featureamount for specifying the natural feature point, in a three-dimensionalworld coordinate system set in a real space, are stored.

FIG. 2 is a view illustrating an example of a marker. A marker 200includes an the identification pattern 201 and a rectangular pattern 202that surrounds the identification pattern 201. The identificationpattern 201 may preferably be any pattern as long as it is easilydistinguished from a design of the surrounding of a place where themarker 200 is installed, for example. In the present embodiment, inorder to estimate a position and an orientation of the image capturingunit 3, positions of the four corners on the outer circumference of therectangular pattern 202 are detected on the image. Further, the markeris not limited to that illustrated in FIG. 2, but any marker as long asit is easily distinguished from a design of the surrounding of a placewhere the marker is installed may be preferably used. Moreover, otherpositions of the marker may preferably be used for the estimation of aposition and an orientation of the image capturing unit 3.

Note that, in the present embodiment, for simplification of the worldcoordinate system set in the real space, an original point is set at thecenter of a marker, an X-axis and a Y-axis are set on a plane where themarker is included, and a Z-axis is set on a normal line of the planewhere the marker is included.

FIG. 3 is a view illustrating an example of a three-dimensional map. Ineach line of a three-dimensional map 300, recorded are three-dimensionalcoordinates of any one of natural feature points or the four corners onthe outer circumference of a marker, a type flag (natural feature point:f and marker: m) indicating whether the marker or the natural featurepoint, and the feature amount of the natural feature point. In thisexample, three-dimensional coordinates and the like of the respectivenatural feature points are recorded in 1st to 64th lines, for example.Moreover, three-dimensional coordinates and the like of the four cornersof the marker are recorded in 65th to 68th lines.

Further, the feature amount of a natural feature point may preferably beany feature amount as long as it may be used to specify the naturalfeature point on an image, and for example, may preferably be aluminance value of a pixel on an image at which the natural featurepoint is positioned and a luminance value of eight vicinity pixels or 24vicinity pixels in the surrounding. Alternatively, the feature amount ofa natural feature point may preferably be a value of each color of apixel on an image at which the natural feature point is positioned oreight vicinity pixels or 24 vicinity pixels in the surrounding. Stillalternatively, the feature amount of a natural feature point maypreferably be a scalar amount or a vector amount that is obtained bycalculation to detect the natural feature point from an image. Forexample, in order to detect natural feature points from an image, whenScale-Invariant Feature Transform (SIFT) is calculated, the featureamount of a natural feature point may preferably be a SIFT value of apixel on an image at which the natural feature point is positioned. Notethat, as for the calculation of an SIFT value and the detection offeature points by SIFT, for example, see David G. Lowe, “DistinctiveImage Features from Scale-Invariant Keypoints”, International Journal ofComputer Vision, Vol. 60, No. 2, pp. 91-110, 2004.

Three-dimensional coordinates of the four corners of a marker and eachnatural feature point are measured in advance in order to create athree-dimensional map before processing of estimating a position and anorientation of the image capturing unit 3 is executed. For example, intwo images obtained in such a manner that the image capturing unit 3takes images of a marker and a surrounding thereof at each of differenttwo points of which position and orientation (optical axis direction)are known in advance, the four corners of the marker and natural featurepoints are respectively detected. At that time, the control unit 6 maypreferably perform feature point detection processing, for example,template matching or corner detection filtering to detect the fourcorners of the marker and the natural feature points. Alternatively, auser may preferably designate the four corners of the marker and thenatural feature points on each of the images.

Next, out of natural feature points detected from each of the images,the natural feature points indicating the same natural feature point areassociated to each other. At that time, the control unit 6 maypreferably determine, for example, natural feature points on the twoimages in which the SIFT calculation values are most corresponded ornatural feature points on the two images that are most corresponded bypattern matching of surrounding regions of the natural feature points onthe images, as to be corresponded to the same natural feature point.

Each pixel on the image indicates an angle relative to the optical axisof the image capturing unit 3. Accordingly, as natural feature pointsdetected from an image acquired at a certain photographed point, adirection from the photographed point to the natural feature point maybe obtained. Accordingly, when positions of pixels on two imagescorresponding to the same natural feature point are respectively known,three-dimensional coordinates of the natural feature point may beobtained in accordance with the principle of triangulation. Similarly,three-dimensional coordinates of the four corners of the marker may beobtained.

Moreover, the control unit 6 may preferably calculate the feature amountof each natural feature point obtained from the three-dimensionalcoordinates, from any of the images in which the natural feature pointis detected, based on a pixel corresponding to the natural feature pointand pixels in the surrounding thereof. Note that, three-dimensionalcoordinates of the four corners of a marker and natural feature pointsmay preferably be obtained by various methods in which from multipleimages, a position of an object picked up on the images is obtained, forexample, in accordance with a method disclosed in Yamada et al., “LatestAlgorithm for 3-D Reconstruction from Two Views”, IPSJ SIG TechnicalReport, vol. 2009-CVIM-168-15, pp. 1-8, 2009.

The control unit 6 includes, one processor or multiple processors andsurrounding circuits thereof. Further, the control unit 6 is connectedto the respective units of the mobile terminal 1 via signal lines, andcontrols the entire mobile terminal 1. Moreover, every time the controlunit 6 receives an image from the image capturing unit 3, the controlunit 6 estimates a position and an orientation of the image capturingunit 3 based on a marker or natural feature points that are picked up onthe image. Note that, hereinafter, estimation processing of a positionand an orientation of the image capturing unit 3 based on a marker isreferred to as marker-based orientation estimation processing.Meanwhile, estimation processing of a position and an orientation of theimage capturing unit 3 based on natural feature points is referred to asnatural feature point-based orientation estimation processing.

FIG. 4 is a functional block diagram of the control unit 6. The controlunit 6 includes a marker detection unit 11, a marker-based orientationestimation unit 12, a shift preparation start determination unit 13, aninitial orientation setting unit 14, a feature point detection unit 15,a feature point-based orientation estimation unit 16, a markerre-detection determination unit 17, and a shift determination unit 18.Each of these units included in the control unit 6 is implemented by,for example, a computer program that is executed on the control unit 6.Further, these respective units included in the control unit 6 maypreferably be mounted on the mobile terminal 1 as an integrated circuitthat implements functions of these respective units, independent of theprocessor included in the control unit 6.

While the control unit 6 executes the marker-based orientationestimation processing, or when it is determined that there is apossibility that the marker-based orientation estimation processing maybe restarted, every time the marker detection unit 11 obtains an imagefrom the image capturing unit 3, the marker detection unit 11 detects amarker from the image.

For that purpose, the marker detection unit 11 performs templatematching on the image using, for example, multiple templatesrepresenting patterns seen from the various directions to detect amarker on the image.

At that time, in order to accelerate the marker detection processing,the marker detection unit 11 may preferably binarize each pixel in theimage depending on whether a luminance value thereof is a predeterminedthreshold value or more. For example, the rectangular pattern 202 of themarker 200 and the identification pattern 201 inside the rectangularpattern 202 illustrated in FIG. 2 are blacker than a surroundingthereof. This results in a luminance value of pixels corresponding tothose patterns lower than a luminance value of pixels in thesurrounding, even on the image. Accordingly, on the binarized image, thepixels corresponding to the patterns included in the marker 200 have apixel value different from that of other pixels. For example, the markerdetection unit 11 makes a luminance value of a pixel on the binarizedimage corresponding to a pixel having a luminance value equal to or morethan a predetermined threshold value relative high, and makes aluminance value of a pixel on the binarized image corresponding to apixel having a luminance value less than the threshold value relativelow. Accordingly, the pixels corresponding to the patterns included inthe marker have a relative low luminance value. Hereinafter, a pixel onthe binarized image having a relative low luminance value is referred toas a black pixel for convenience.

The marker detection unit 11 executes labeling processing on a set ofblack pixels to obtain one or more of a black pixel region that is aregion in which the black pixels are connected to one another. Further,the marker detection unit 11 performs edge following on each black pixelregion to obtain an edge of each black pixel region. In addition, themarker detection unit 11 performs broken line approximation on each edgeto set a black pixel region in which the edge may be approximated withfour lines as a marker candidate region. The marker detection unit 11performs template matching between the marker candidate region and atemplate, and calculates, for example, a normalized cross-correlationvalue between the marker candidate region and the template in accordancewith the following expression.

$\begin{matrix}{R = \frac{\sum\limits_{j = 0}^{N - 1}\; {\sum\limits_{i = 0}^{M - 1}\; \left\{ {\left( {{I\left( {i,j} \right)} \cdot I_{av}} \right)\left( {{T\left( {i,j} \right)} - T_{av}} \right)} \right\}}}{\sqrt{\left\{ {\sum\limits_{j = 0}^{N - 1}\; {\sum\limits_{i = 0}^{M - 1}\; \left( {{I\left( {i,j} \right)} - I_{av}} \right)^{2}}} \right\} \left\{ {\sum\limits_{j = 0}^{N - 1}\; {\sum\limits_{i = 0}^{M - 1}\; \left( {{T\left( {i,j} \right)} - T_{av}} \right)^{2}}} \right\}}}} & (1)\end{matrix}$

Herein, T (i, j) represents a luminance value of a pixel (i, j) of thetemplate, and I (i, j) represents a luminance value of a pixel (i, j) inthe marker candidate region. Further, Tav represents a luminance meanvalue of the template, and Iav represents a luminance mean value of themarker candidate region.

When the maximum value of the normalized cross-correlation value becomesa marker detection threshold value (for example, 0.8) or more, themarker detection unit 11 determines that a marker is picked up in amarker candidate region corresponding to the maximum value of thenormalized cross-correlation value. Further, the marker detection unit11 sets coordinates of the four corners of the marker candidate regionas coordinates of the marker on the image.

In contrast, when the maximum value of the normalized cross-correlationvalue is less than the marker detection threshold value, the markerdetection unit 11 determines that a marker is not picked up in themarker candidate region.

Further, when the marker re-detection determination unit 17 limits asearch region of a marker, the marker detection unit 11 executesdetection processing of the marker within the limited search region.Note that, details of the marker re-detection determination unit 17 anda search region of a marker are described later. Moreover, the markerdetection unit 11 may preferably set a region in which a circumscribedrectangle of a region surrounded by the four corners of a marker on animage obtained immediately prior is expanded by a predetermined offsetin the vertical direction and in the horizontal direction, as a markersearch region with respect to a current image, and perform markerdetection processing within the marker search region.

Every time the marker detection unit 11 obtains coordinates of the fourcorners of a marker on an image, the marker detection unit 11 stores thecoordinates in the storage unit 5 so as to indicate an acquisition orderof the corresponding image.

Every time a marker is detected form the image obtained by the imagecapturing unit 3, the marker-based orientation estimation unit 12estimates a position and an orientation of the image capturing unit 3based on the marker.

The marker-based orientation estimation unit 12 estimates, in order toestimate a position and an orientation of the image capturing unit 3,for example, a conversion matrix from a world coordinate system in whicha marker is used as the reference to a camera coordinate system in whichthe image capturing unit 3 is used as the reference.

The conversion matrix is represented by the following expression.

$\begin{matrix}{\begin{bmatrix}X_{c} \\Y_{c} \\Z_{c} \\1\end{bmatrix} = {{\begin{pmatrix}\; & R & \; & T \\0 & 0 & 0 & 1\end{pmatrix}\begin{bmatrix}X_{m} \\Y_{m} \\Z_{m} \\1\end{bmatrix}} = {\begin{bmatrix}R_{11} & R_{12} & R_{13} & T_{1} \\R_{21} & R_{22} & R_{23} & T_{2} \\R_{31} & R_{32} & R_{33} & T_{3} \\0 & 0 & 0 & 1\end{bmatrix}\begin{bmatrix}X_{m} \\Y_{m} \\Z_{m} \\1\end{bmatrix}}}} & (2)\end{matrix}$

Herein, (Xc, Yc, Zc) represents three-dimensional coordinates in thecamera coordinate system that correspond to points of three-dimensionalcoordinates (Xm, Ym, Zm) in the world coordinate system in which amarker is used as the reference. Moreover, a matrix R represents arotation movement component and a matrix T represents a parallelmovement component. Further, a Z-axis in the camera coordinate system isset in parallel with an optical axis of the image capturing unit 3.

Moreover, a relation between the camera coordinate system and acoordinate system on an image to be created by the image capturing unit3 is represented in accordance with a perspective transformation modelby the following expression.

$\begin{matrix}{\begin{bmatrix}{xc} \\{yc} \\h \\1\end{bmatrix} = {{P\begin{bmatrix}X_{c} \\Y_{c} \\Z_{c} \\1\end{bmatrix}} = {\begin{bmatrix}P_{11} & P_{12} & P_{13} & 0 \\0 & P_{22} & P_{23} & 0 \\0 & 0 & 1 & 0 \\0 & 0 & 0 & 1\end{bmatrix}\begin{bmatrix}X_{c} \\Y_{c} \\Z_{c} \\1\end{bmatrix}}}} & (3)\end{matrix}$

Herein, (xc, yc) represent coordinates on the image that correspond topoints of (X_(c), Y_(c), Z_(c)) on the camera coordinate system.

In order to estimate the matrix R representing the rotation movementcomponent, the marker-based orientation estimation unit 12 obtains aline segment that connects two points among the four corners of themarker to calculate unit direction vectors V1, V2 of two sides of themarker that are opposed to each other.

An expression of the two sides of the marker that are opposed to eachother is represented by the following expression.

a ₁ x+b ₁ y+c ₁=0

a ₂ x+b ₂ y+c ₂=0  (4)

Further, the following expression may be obtained substitution of (xc,yc) in Expression 3 into Expression 4.

a ₁ P ₁₁ X _(c)+(a ₁ P ₁₂ +b ₁ P ₂₂)Y _(c)+(a ₁ P ₁₃ +b ₁ P ₂₃ +c ₁)Z_(c)=0

a ₂ P ₁₁ X _(c)+(a ₂ P ₁₂ +b ₂ P ₂₂)Y _(c)+(a ₂ P ₁₃ +b ₂ P ₂₃ +c ₂)Z_(c)=0  (5)

The Expression 5 is an equation representing planes in which the twosides of the marker that are opposed to each other are present, in thecamera coordinate system. The two sides of the marker that are opposedto each other are in parallel with each other. Thus, direction vectorsof the two sides are consistent with each other, and become an in-planedirection of the two plane represented by Expression 5. Accordingly,outer products of normal line vectors of the two planes in Expression 5respectively become unit direction vectors V1, V2 of the two sides.

In addition, the marker-based orientation estimation unit 12 calculatesan outer product, that is “vector product”, of the unit directionvectors V1 and V2 to calculate a unit direction vector V3 in a directionvertical to the marker plane. The matrix R representing the rotationmovement component is represented as [V1 ^(t)V2 ^(t)V3 ^(t)].

Moreover, the marker-based orientation estimation unit 12 couplesExpression 2 and Expression 3 in order to estimate the matrix Trepresenting the parallel movement component. Further, the marker-basedorientation estimation unit 12 inputs coordinates of each of the fourcorners of the marker on the image and coordinates thereof in the worldcoordinate system into the coupled expression. This allows themarker-based orientation estimation unit 12 to acquire eightsimultaneous equations for three elements [T1, T2, T3] of the matrix T.Consequently, the marker-based orientation estimation unit 12 solves thesimultaneous equations by a least-squares method to calculate respectiveelements [T1, T2, T3] of the matrix T.

In order to reduce an error by calculation of the rotation movementcomponent, the marker-based orientation estimation unit 12 convertsrespective components of the matrix R so as to be expressed as threerotation angles in the Z-axis of the world coordinate system in thecamera coordinate system, in accordance with Rodrigues formula.

The marker-based orientation estimation unit 12 obtains coordinates inthe world coordinate system corresponding to a position of the originalpoint in the camera coordinate system, in the acquired conversionmatrix. This allows the marker-based orientation estimation unit 12 toestimate a position of the image capturing unit 3. Moreover, themarker-based orientation estimation unit 12 obtains, in the acquiredconversion matrix, a direction of the world coordinate systemcorresponding to an axis Zc (in other words, optical axis) in the cameracoordinate system. This allows the marker-based orientation estimationunit 12 to estimate an orientation of the image capturing unit 3, inother words, the optical axis direction of the image capturing unit 3 inthe world coordinate system.

The marker-based orientation estimation unit 12 stores the estimatedcoordinates in the world coordinate system representing the position ofthe image capturing unit 3 and the estimated unit direction vector inthe world coordinate system representing the orientation of the imagecapturing unit 3, in the storage unit 5.

FIG. 5 illustrates an example of a list indicating a position and anorientation of the image capturing unit 3 for every photographing timestored in the storage unit 5. In each line of a list 500, recorded aresix-dimensional vector (x, y, z, rx, ry, rz) indicating a position andan orientation of the image capturing unit 3 in the world coordinatesystem and coordinates of the four corners of a marker on an image.Further, elements x, y, z of the six-dimensional vector indicating aposition and an orientation of the image capturing unit 3 respectivelyrepresent coordinates in the horizontal direction and coordinates in thevertical direction of the marker plane, and coordinates in the normalline direction of the marker plane. Moreover, elements rx, ry, rzrespectively represent angles made by the optical axis of the imagecapturing unit 3 in the horizontal direction and in the verticaldirection with respect to the normal line direction of the marker plane,and a rotation angle of the image capturing unit 3 around the normalline direction of the marker plane.

While the marker-based orientation estimation processing is performed,every time an image is acquired, the shift preparation startdetermination unit 13 calculates a failure index indicating apossibility that a marker may not be detected from the image.

FIG. 6A to FIG. 6D are views each illustrating a case where there is apossibility that a marker may not be detected from an image. An exampleillustrated in FIG. 6A represents a case where on an image 600, a marker601 is moved toward an end of the image, and it is assumed that themarker 601 come out from the image 600 in the future. Moreover, in anexample illustrated in FIG. 6B, because a normal line n of a marker 611is largely inclined to an optical axis direction OA of the imagecapturing unit 3 to cause distortion of the shape of the marker on theimage, it is assumed that the marker is difficult to be detected fromthe image.

In addition, in an example illustrated in FIG. 6C, the image capturingunit 3 is moved away from a marker to make a marker 622 on a currentimage smaller than a marker 621 on an image that is acquired immediatelyprior. From this, it is assumed that the significantly reduced size ofthe marker on the image 600 makes it difficult to detect the marker inthe future. Moreover, an example illustrated in FIG. 6D represents timechange in reliability of marker detection, and the horizontal axisrepresents time and the longitudinal axis represents reliability. Inthis example, a curve 631 indicating a relation between the time and thereliability is lowered as the time elapses, and in such a case, it isassumed that a marker may not be accurately detected.

Consequently, the shift preparation start determination unit 13calculates, as a failure index, for example, at least one out of atraveling speed and a current position of coordinates of the fourcorners of a marker, the inclination of the normal line of the markerwith respect to the optical axis of the image capturing unit 3, an areaof the marker on the image, and reliability of the marker detection.

When at least one failure index out of the abovementioned multiplefailure indices satisfies a shift preparation start reference, the shiftpreparation start determination unit 13 determines that preparation forshifting orientation estimation processing from marker-based orientationestimation processing to natural feature point-based orientationestimation processing is started. Alternatively, when at least onefailure index out of the abovementioned multiple failure indicessatisfies the shift preparation start reference over multiple frames,the shift preparation start determination unit 13 may preferablydetermine that preparation for shifting orientation estimationprocessing from marker-based orientation estimation processing tonatural feature point-based orientation estimation processing isstarted. Still alternatively, when two or more indices out of theabovementioned multiple failure indices satisfy the shift preparationstart reference, the shift preparation start determination unit 13 maypreferably determine that preparation for shifting orientationestimation processing from marker-based orientation estimationprocessing to natural feature point-based orientation estimationprocessing is started.

For example, the shift preparation start determination unit 13calculates the traveling speed and the movement direction of each of thefour corners of a marker by applying linear interpolation or splineinterpolation based on the changing amount of coordinates of the fourcorners of the marker on the image in the past several frames and thephotographing cycle in the past several frames. Further, the shiftpreparation start determination unit 13 determines that the shiftpreparation start reference is satisfied when the number of pixels fromany image end to the center of gravity of the coordinates of the fourcorners of the marker (hereinafter, simply referred to as marker gravitycenter) is a predetermined number of pixels (for example, 50 pixels) orless. Alternatively, the shift preparation start determination unit 13may preferably determine that the shift preparation start reference issatisfied when a movement direction of the marker gravity center is adirection apart from the center of the image and the traveling speed isa predetermined speed (for example, 5 pixels/frame) or more. Moreover,the shift preparation start determination unit 13 may preferablydetermine that the shift preparation start reference is satisfied whenthe number of pixels from the marker gravity center to an image end thatis located in the destination of the movement direction of the markergravity center is a predetermined number of pixels or less and thetraveling speed of the marker gravity center is a predetermined speed ormore. In this case, the predetermined number of pixels and thepredetermined speed are set to values in which a part of the marker isassumed to come off from the image after from several frames to severaltens of frames. For example, when the predetermined number of pixels ism pixels (m is an integer of one or more), the predetermined speed maybe set to (m/10) pixel/frame.

Moreover, when an angle made between the optical axis of the imagecapturing unit 3 and the normal line of the marker becomes too large,there is a possibility that the control unit 6 may not detect the markerfrom the image. Consequently, the shift preparation start determinationunit 13 determines that the shift preparation start reference issatisfied when the angle made between the optical axis of the imagecapturing unit 3 and the normal line of the marker becomes apredetermined angle (for example, 70°) or more.

In addition, when the area of the marker on the image is too small, thecontrol unit 6 may not detect the marker from the image. Consequently,the shift preparation start determination unit 13 determines that theshift preparation start reference is satisfied when the number of pixelsincluded in a quadrangle surrounded by the four corners of the marker onthe image (in other words, the area of the marker on the image) is apredetermined area value or less. Further, the predetermined area valueis set, for example, as a value obtained such that a minimum size of amarker that allows a pattern of the marker on the image to be identifiedis multiplied by a safety coefficient (for example, 1.1 to 1.3), to 400,for example.

Further, the shift preparation start determination unit 13 maypreferably estimate whether or not the area of the marker decreases inan image in a next frame by applying, for example, linear interpolationbased on change in area of the marker on the image in the past severalframes. Further, the shift preparation start determination unit 13 maypreferably determine that the shift preparation start reference issatisfied when the area of the marker in an image of a current frame isthe predetermined area value or less and the area of the marker isestimated to decrease in a next frame.

In addition, when reliability relating to the detection is low at a timeof detecting a marker from an image, there is a possibility that thecontrol unit 6 may not detect the marker from the image. Moreover, thereliability relating to the detection may be a maximum value of anormalized cross-correlation value between the image and the templatethat is calculated by the marker detection unit 11. Consequently, theshift preparation start determination unit 13 determines that the shiftpreparation start reference is satisfied when the maximum value of thenormalized cross-correlation value is a predetermined reliabilitythreshold value or less. Further, the predetermined reliabilitythreshold value is set to a value higher than the marker detectionthreshold value, for example, to 0.9.

Alternatively, the shift preparation start determination unit 13 maypreferably assign weights to the abovementioned multiple indices, anddetermine whether or not preparation for shifting orientation estimationprocessing from marker-based orientation estimation processing tonatural feature point-based orientation estimation processing isstarted. For example, when the traveling speed of the image capturingunit 3 is high and a possibility that a marker comes off outside theimage is high, when a shift preparation start reference relating to themarker gravity center with respect to one image is satisfied, the shiftpreparation start determination unit 13 determines that shiftpreparation to natural feature point-based orientation estimationprocessing is immediately started. Meanwhile, as for the marker area,the optical axis angle, and the reliability relating to the detection,when the shift preparation start references relating to two or moreamong those are satisfied, the shift preparation start determinationunit 13 determines that shift preparation to natural feature point-basedorientation estimation processing is started. Alternatively, as for themarker area, the optical axis angle, and the reliability relating to thedetection, when the shift preparation start reference is satisfied withrespect to multiple continuous images, the shift preparation startdetermination unit 13 may preferably determine that shift preparation tothe natural feature point-based orientation estimation processing isstarted.

When determining that the shift preparation to the natural featurepoint-based orientation estimation processing is started, the shiftpreparation start determination unit 13 notifies the initial orientationsetting unit 14 of the determination.

Moreover, with respect to an image that is acquired after the shiftpreparation to the natural feature point-based orientation estimationprocessing is started, when the shift preparation start reference is notsatisfied as for any of the abovementioned failure indices, the shiftpreparation start determination unit 13 may preferably notify theinitial orientation setting unit 14 of stop of the shift preparation.Alternatively, with respect to continuous multiple images, when theshift preparation start reference is not satisfied as for any of theabovementioned failure indices, the shift preparation startdetermination unit 13 may preferably notify the initial orientationsetting unit 14 of stop of the shift preparation.

Every time an image is acquired after the initial orientation settingunit 14 is notified from the shift preparation start determination unit13 that the preparation for shifting the orientation estimationprocessing to the natural feature point-based orientation estimationprocessing is started, the initial orientation setting unit 14determines a position and an orientation of the image capturing unit 3to be initial values when the natural feature point-based orientationestimation processing is started.

The initial orientation setting unit 14 applies a prediction filter to,for example, the position and the orientation of the image capturingunit 3 for the past several frames from a point determined that thepreparation for shifting to the natural feature point-based orientationestimation processing is started, to obtain estimated values of theposition and the orientation of the image capturing unit 3 when a nextimage is acquired. The estimated values may be used as initial valueswhen natural feature point-based orientation estimation processing isstarted in a case where a marker is not detected from a next image. Withthis, the initial orientation setting unit 14 may use a position and anorientation of the image capturing unit 3 that are estimated based on amarker on the image when the detection accuracy of the marker is high,as initial values when the natural feature point-based orientationestimation processing is started. This allows the estimation accuracy ofthe initial values to be improved. In the present embodiment, theinitial orientation setting unit 14 uses a particle filter as theprediction filter.

The initial orientation setting unit 14 repeats the following processingfor each image in order from the oldest image.

(1) Firstly, the initial orientation setting unit 14 creates a pluralityof particles each having a six-dimensional state amount (x, y, z, rx,ry, rz) indicating a position and an orientation of the image capturingunit 3 and a likelihood of the state amount on a random basis. Further,the initial orientation setting unit 14 leaves the state amount ofremaining particles unchanged after the second turn. Further, theinitial orientation setting unit 14 creates particles newly before thenumber of particles reaches a predetermined number. Moreover, thelikelihood of the respective particles is identical.

(2) Next, the initial orientation setting unit 14 projects the fourcorners of a marker onto an image plane in accordance with Expression 2and Expression 3 for each particle corresponding to a temporary positionand a temporary orientation of the image capturing unit 3 that areexpressed by the state amount included in the particle. Then, theinitial orientation setting unit 14 obtains coordinates of a projectionpoint of each of the four corners of the marker on the image plane.

(3) The initial orientation setting unit 14 obtains, for each particleand each of the four corners of the marker, coordinates of theprojection point and a distance between the coordinates of thecorresponding corners of the maker that are detected from the image, andcalculates a likelihood of the particle based on a mean value of thedistances. For example, the initial orientation setting unit 14 sets areciprocal number of a number in which 1 is added to the mean value ofthe distances as the likelihood. Alternatively, the initial orientationsetting unit 14 may preferably calculate a feature amount, for each ofthe four corners of the marker, by performing feature point detectioncalculation such as SIFT to each projection point on the image and eachpoint detected from the image, and set a reciprocal number of the errorsum of squares between the feature amounts as a likelihood.

(4) The initial orientation setting unit 14 erases a particle of whichlikelihood is a predetermined threshold value (for example, 0.1) orless.

(5) The initial orientation setting unit 14 weights and averages thestate amount of remaining particles with the likelihood of the particlesto obtain estimated values of the position and the orientation of theimage capturing unit 3.

The initial orientation setting unit 14 acquires the estimated values ofthe position and the orientation of the image capturing unit 3 byrepeating the abovementioned processing before a current image isacquired. The estimated values are stored as initial values of theposition and the orientation of the image capturing unit 3 when a nextimage is acquired. Further, the initial orientation setting unit 14 maypreferably store the state amount of each particle remaining in thecurrent image in the storage unit 5. Further, when the marker also isdetected from a next image, the initial orientation setting unit 14 maypreferably use the stored state amount of each particle to execute theabovementioned processing (1) to processing (5) only once, and updatethe initial values of the position and the orientation of the imagecapturing unit 3.

Further, the initial orientation setting unit 14 may preferably use, forsetting initial values of the position and the orientation of the imagecapturing unit 3, other a nonlinear prediction filter such as a Kalmanfilter or a linear prediction filter, instead of the particle filter.

Moreover, when the initial orientation setting unit 14 is notified fromthe shift preparation start determination unit 13 that the preparationfor shifting the orientation estimation processing to the naturalfeature point-based orientation estimation processing is stopped, theinitial orientation setting unit 14 terminates the setting of initialvalues.

FIG. 7 is a conceptual diagram of initial value prediction in thenatural feature point-based orientation estimation processing. In FIG.7, the horizontal axis represents time. In this example, it is assumedthat the initial value prediction by the natural feature point-basedorientation estimation processing is not performed before time t1, theinitial value prediction by the natural feature point-based orientationestimation processing is performed during a period from the time t1 totime t2, and the natural feature point-based orientation estimationprocessing is performed after the time t2.

Moreover, in FIG. 7, quadrangles 701 each indicate a position and anorientation of the image capturing unit 3 that are estimated based onthe marker and stored in the storage unit 5. In particular, a blackquadrangle 701 a indicates a position and an orientation of the imagecapturing unit 3 that are estimated based on the marker detected from acurrent image. Moreover, arrows 702 indicate that estimation of theposition and the orientation of the image capturing unit 3 by using theprediction filter is executed. Further, a circle 703 indicates aposition and an orientation of the image capturing unit 3 that areestimated using the prediction filter.

Before the time t1, as illustrated in the upper most line, the initialvalue prediction by the prediction filter is not performed, and thepositions and the orientations of the image capturing unit 3 that areestimated based on the marker during a period from the current time to alatest certain period 710 are stored. Further, as illustrated in thesecond line, when the initial value prediction is started at the timet1, the estimation of the position and the orientation of the imagecapturing unit 3 using the prediction filter is executed, and anestimated value 703 is stored in the storage unit 5.

Thereafter, before it reaches the time t2, as illustrated in the thirdline, every time an image is acquired, processing by the predictionfilter is performed once to update the estimated value 703. Moreover,the position and the orientation of the image capturing unit 3 that areestimated based on the marker detected from the image are also stored.Further, as illustrated in the lower most line, when the marker is notdetected from a current image at the time t2, the estimated value 703that is calculated when the immediately prior image is acquired is usedas initial values of the natural feature point-based orientationestimation processing.

The feature point detection unit 15 detects, every time an image isacquired from the image capturing unit 3, natural feature points fromthe image, while the control unit 6 executes the natural featurepoint-based orientation estimation processing. Accordingly, the featurepoint detection unit 15 performs, for example, corner detectionfiltering processing or SIFT calculation processing on each pixel in theimage to calculate the feature amount indicating a natural feature pointfor each pixel. Further, the feature point detection unit 15 detects, asthe natural feature point, a pixel in which a difference with thefeature amount of any of natural feature points recorded in thethree-dimensional map is less than a predetermined value.

The feature point-based orientation estimation unit 16 estimates aposition and an orientation of the image capturing unit 3 based onnatural feature points. Accordingly, the feature point-based orientationestimation unit 16 estimates the rotation movement component and theparallel movement component in Expression 2 using a position and anorientation of the image capturing unit 3 that are estimated based onthe immediately prior image as initial values, based on feature pointsdetected from the image. Further, immediately after the estimationprocessing is shifted from the marker-based orientation estimationprocessing to the natural feature point-based orientation estimationprocessing, the feature point-based orientation estimation unit 16estimates the rotation movement component and the parallel movementcomponent using, as the initial values, the position and the orientationof the image capturing unit 3 that are estimated by applying theprediction filter.

FIG. 8 is a conceptual diagram of natural feature point-basedorientation estimation processing. The feature point-based orientationestimation unit 16 projects, in accordance with Expression 2 andExpression 3, respective natural feature points 801 onto an image plane800, and obtains coordinates of each of projection points 802 on theimage plane 800 corresponding to coordinates of each of the naturalfeature points in the world coordinate system. Then, the featurepoint-based orientation estimation unit 16 calculates, for each naturalfeature point, the sum of squares of a distance between the projectionpoint 802 and a corresponding natural feature point 803 detected fromthe image, as an evaluation value.

The feature point-based orientation estimation unit 16 obtains theevaluation value while correcting the respective elements of therotation movement component and the parallel movement component inaccordance with, for example, a steepest-descent method. Then, thefeature point-based orientation estimation unit 16 obtains estimatedvalues of the position and the orientation of the image capturing unit 3in accordance with the rotation movement component and the parallelmovement component when the evaluation value becomes minimum.

Further, the feature point-based orientation estimation unit 16 maypreferably estimate a position and an orientation of the image capturingunit 3 in accordance with other methods in which a position and anorientation of a camera are estimated based on multiple feature points.For example, the feature point-based orientation estimation unit 16 maypreferably estimate a position and an orientation of the image capturingunit 3 in accordance with a method disclosed in G. Klein et al.,“Parallel Tracking and Mapping for Small AR Workspaces”, in Proceedingsof 6^(th) IEEE and ACM International Symposium on Mixed and AugmentedReality, Nara, 2007.

In this manner, in the present embodiment, the feature point-basedorientation estimation unit 16 determines initial values, when theestimation processing is shifted from the marker-based orientationestimation processing to the natural feature point-based orientationestimation processing, by using the position and the orientation of theimage capturing unit 3 that are estimated based on a marker when themarker detection accuracy is high. Accordingly, the initial values arehighly likely to be close to the actual position and the actualorientation of the image capturing unit 3, so that the featurepoint-based orientation estimation unit 16 is highly likely to be ableto obtain the minimum value of the evaluation value without falling in alocal minimum of the evaluation value that is acquired when the initialvalues are different from the original position and the originalorientation of the image capturing unit 3. Accordingly, the featurepoint-based orientation estimation unit 16 may improve the estimationaccuracy of the position and the orientation of the image capturing unit3 when the estimation processing is shifted from the marker-basedorientation estimation processing to the natural feature point-basedorientation estimation processing.

The feature point-based orientation estimation unit 16 storescoordinates in the world coordinate system representing the position ofthe image capturing unit 3 and the unit direction vector in the worldcoordinate system representing the orientation of the image capturingunit 3 that are estimated, in the storage unit 5.

The marker re-detection determination unit 17 calculates, while thenatural feature point-based orientation estimation processing isexecuted, every time an image is acquired, a success index representinga possibility that a marker is detected from the image. The markerre-detection determination unit 17 calculates, as a success index, forexample, at least one out of the marker gravity center, the inclinationof the normal line of the marker with respect to the optical axis of theimage capturing unit 3, and the area of the marker on the image.

Further, the marker is not detected at the present. Accordingly, themarker re-detection determination unit 17 projects coordinates of thefour corners of the marker recorded in the three-dimensional map ontothe image plane, based on the position and the orientation of the imagecapturing unit 3 estimated by the feature point-based orientationestimation unit 16, in accordance with Expression 2 and Expression 3.Then, the marker re-detection determination unit 17 calculatescoordinates of the marker gravity center and the area of the marker fromcoordinates of the four corners of the marker on the image plane.Similarly, the marker re-detection determination unit 17 calculates theinclination of the normal line of the marker with respect to the opticalaxis of the image capturing unit 3 based on the orientation of the imagecapturing unit 3 estimated by the feature point-based orientationestimation unit 16.

When at least one out of the abovementioned multiple success indicessatisfies a re-shift preparation start reference, the markerre-detection determination unit 17 determines that preparation forshifting orientation estimation processing from marker-based orientationestimation processing to natural feature point-based orientationestimation processing is started. Alternatively, over multiple frames,when at least one out of the abovementioned multiple success indicessatisfies the re-shift preparation start reference, the shiftpreparation start determination unit 13 may preferably determine thatpreparation for shifting orientation estimation processing frommarker-based orientation estimation processing to natural featurepoint-based orientation estimation processing is started. Stillalternatively, two or more out of the abovementioned multiple successindices satisfy the re-shift preparation start reference, the shiftpreparation start determination unit 13 may preferably determine thatpreparation for shifting orientation estimation processing frommarker-based orientation estimation processing to natural featurepoint-based orientation estimation processing is started.

The marker re-detection determination unit 17 determines that there-shift preparation start reference is satisfied when the number ofpixels from the marker gravity center to the closest image end is apredetermined number of pixels (for example, 50 pixels) or more.Alternatively, the marker re-detection determination unit 17 maypreferably determine that the re-shift preparation start reference issatisfied when the inclination of the normal line of the marker withrespect to the optical axis of the image capturing unit 3 is apredetermined angle (for example, 80°) or less. Still alternatively, themarker re-detection determination unit 17 may preferably determine thatthe re-shift preparation start reference is satisfied when the area ofthe marker is a predetermined area threshold value (for example, 100pixels) or more.

Alternatively, the marker re-detection determination unit 17 maypreferably assign weights to the abovementioned multiple successindices, and determine whether or not preparation for shifting theorientation estimation processing to the marker-based orientationestimation processing is started. For example, when the traveling speedof the image capturing unit 3 is high and the entire marker is highlylikely to be picked up in an image, the marker re-detectiondetermination unit 17 causes the marker detection unit 11 to immediatelyexecute marker detection when the re-shift preparation start referenceas for the marker gravity center with respect to one image is satisfied.Meanwhile, as for the marker area and the optical axis angle, whenre-shift preparation start references as for the both, the markerre-detection determination unit 17 causes the marker detection unit 11to immediately execute marker detection. Alternatively, as for themarker area and the optical axis angle, when the re-shift preparationstart reference is satisfied with respect to continuous multiple images,the marker re-detection determination unit 17 may preferably cause themarker detection unit 11 to execute marker detection.

When determining that preparation for shifting orientation estimationprocessing from marker-based orientation estimation processing tonatural feature point-based orientation estimation processing is startedwith respect to a current image, the marker re-detection determinationunit 17 causes the marker detection unit 11 to detect a marker from thecurrent image. At that time, for improving the detection accuracy of themarker and restricting the amount of calculation, the markerre-detection determination unit 17 may preferably limit the search rangeof the marker. For example, the marker re-detection determination unit17 sets a region that includes the marker projected on to an image planeand is smaller than the image as the marker search region. Specifically,the marker re-detection determination unit 17 obtains a circumscribedrectangle of a region surrounded by lines connecting the four corners ofthe marker on an image plane acquired such that coordinates of the fourcorners in the world coordinate system are projected onto the imageplane. Further, when a left upper end pixel of the circumscribedrectangle is (u, v) and a right lower end pixel of the circumscribedrectangle is (u+w, v+h), the marker re-detection determination unit 17sets a left upper end pixel in the search range of the marker to (u−m,v−m) and a right lower end pixel in the search range of the marker to(u+w+m, v+h+m). Note that m is an offset value, and is set to 10 pixels,for example. Further, when the left upper end pixel (u−m, v−m) in thesearch range of the marker is outside the image, the marker re-detectiondetermination unit 17 may preferably set the left end or the upper endin the search range as the left end or the upper end of the image.Similarly, when the right lower end pixel (u+w+m, v+h+m) in the searchrange of the marker is outside the image, the marker re-detectiondetermination unit 17 may preferably set the right end or the lower endin the search range as the right end or the lower end of the image. Themarker re-detection determination unit 17 notifies the marker detectionunit 11 of the search range of the marker. Further, the markerre-detection determination unit 17 causes the marker detection unit 11to execute marker detection processing within the search range.

While the marker-based orientation estimation processing is executed,when the marker is not detected from an image, the shift determinationunit 18 determines that the orientation estimation processing is shiftedto the natural feature point-based orientation estimation processing.Further, the shift determination unit 18 stops the marker detection unit11 and the marker-based orientation estimation unit 12, and starts upthe feature point detection unit 15 and the feature point-basedorientation estimation unit 16. Further, when the marker detection unit11 may not detect all the four corners of the marker, the shiftdetermination unit 18 determines that the marker is not detected.

Meanwhile, while the natural feature point-based orientation estimationprocessing is executed, when the marker may be detected from an image,the shift determination unit 18 determines that the orientationestimation processing is shifted to the marker-based orientationestimation processing. Further, the shift determination unit 18 stopsthe feature point detection unit 15 and the feature point-basedorientation estimation unit 16, and starts up the marker detection unit11 and the marker-based orientation estimation unit 12. Further, whenthe marker is detected from an image over continuous several frames, theshift determination unit 18 may preferably determine that theorientation estimation processing is shifted to the marker-basedorientation estimation processing.

FIG. 9 is an operation flowchart of orientation estimation processing.Every time the control unit 6 acquires an image from the image capturingunit 3, the control unit 6 executes the orientation estimationprocessing in accordance with the following operation flowchart.

The control unit 6 determines whether or not orientation estimationprocessing that is currently employed is marker-based orientationestimation processing (step S101). When the orientation estimationprocessing that is currently employed is marker-based orientationestimation processing (step S101-Yes), the marker detection unit 11detects a marker from an image (step S102). The shift determination unit18 determines whether or not the marker is detected from an image (stepS103). When the marker is detected from an image (step S103-Yes), themarker-based orientation estimation unit 12 estimates a position and anorientation of the image capturing unit 3 based on a position of themarker on the image (step S104).

Moreover, the shift preparation start determination unit 13 calculatesat least one failure index, and determines whether or not the failureindex satisfies a shift preparation start reference (step S105). Whenthe shift preparation start reference is satisfied (step S105—Yes), theinitial orientation setting unit 14 estimates a position and anorientation of the image capturing unit 3 when a next image is acquiredbased on the estimated values of the position and the orientation of theimage capturing unit 3 in the past several frames. Further, the initialorientation setting unit 14 sets the position and the orientation thatare estimated as initial values of the position and the orientation ofthe image capturing unit 3 when natural feature point-based orientationestimation processing is started (step S106). Further, the initialorientation setting unit 14 stores the initial values in the storageunit 5.

After step S106 or when the shift preparation start reference is notsatisfied at step S105 (step S105—No), the control unit 6 ends theorientation estimation processing.

In contrast, at step S103, when the marker is not detected from an image(step S103—No), the shift determination unit 18 determines that theorientation estimation processing is shifted to the natural featurepoint-based orientation estimation processing, and starts up the featurepoint detection unit 15 and the feature point-based orientationestimation unit 16 (step S107).

After step S107 or when the orientation estimation processing that iscurrently employed is natural feature point-based orientation estimationprocessing at step S101 (step S101—No), the feature point detection unit15 detects natural feature points from an image (step S108). Further,the feature point-based orientation estimation unit 16 sets estimatedvalues of the position and the orientation of the image capturing unit 3when an immediately preceding image is acquired as initial values, andestimates a position and an orientation of the image capturing unit 3based on the detected natural feature points (step S109).

The marker re-detection determination unit 17 calculates at least onesuccess index, and determines whether or not the success index satisfiesa re-shift preparation start reference (step S110). When the re-shiftpreparation start reference is not satisfied (step S110—No), the controlunit 6 ends the orientation estimation processing.

In contrast, when the re-shift preparation start reference is satisfied(step S110—Yes), the marker re-detection determination unit 17 sets amarker search range (step S111). Then, the marker detection unit 11detects the marker within the marker search range (step S112). The shiftdetermination unit 18 determines whether or not the marker is detectedfrom the image (step S113). When the marker is detected from an image(step S113—Yes), the shift determination unit 18 determines that theorientation estimation processing is shifted to the marker-basedorientation estimation processing, starts up the marker detection unit11 and the marker-based orientation estimation unit 12 (step S114).

After step S114 or when the marker is not detected from the image atstep S113 (step S113—No), the control unit 6 ends the orientationestimation processing.

As describe above, while a marker may be detected from an image, theorientation estimation apparatus estimates a position and an orientationof the image capturing unit by the marker-based orientation estimationprocessing to restrict the amount of calculation. Meanwhile, when themarker may not be detected from an image, the orientation estimationapparatus estimates a position and an orientation of the image capturingunit by the natural feature point-based orientation estimationprocessing. This allows the orientation estimation apparatus to estimatea position and an orientation of the image capturing unit even when themarker may not be detected. In addition, when a possibility that markerdetection is failed becomes high, the orientation estimation apparatususes a position and an orientation of the image capturing unit that areacquired by the marker-based orientation estimation processing when thedetection accuracy of the marker is high to estimate a position and anorientation of the image capturing unit when the marker detection isfailed. Further, the orientation estimation apparatus uses the estimatedvalues as initial values as the position and the orientation of theimage capturing unit when the natural feature point-based orientationestimation processing is started. Accordingly, the orientationestimation apparatus may restrict lowering of the estimation accuracy ofthe position and the orientation of the image capturing unit when theorientation estimation processing is shifted from the marker-basedorientation estimation processing to the natural feature point-basedorientation estimation processing.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiments of the presentinvention have been described in detail, it should be understood thatthe various changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

What is claimed is:
 1. An orientation estimation apparatus, comprising:an image capturing unit that creates an image of a photographed range ata predetermined photographing cycle; a storage unit that stores thereina position of a marker installed in advance in a real space andpositions of a plurality of feature points in a surrounding of themarker in the real space; a marker detection unit that detects themarker from the image; a marker-based orientation estimation unit thatestimates, based on a position of the marker on the image and theposition of the marker in the real space, a position and an orientationof the image capturing unit; a feature point detection unit that detectsthe plurality of feature points from the image; a feature point-basedorientation estimation unit that estimates the position and theorientation of the image capturing unit based on positions of theplurality of feature points on the image, the positions of the pluralityof feature points in the real space, and initial values of the positionand the orientation of the image capturing unit; a shift determinationunit that causes, when the marker is not detected from the image, thefeature point detection unit to detect the plurality of feature pointswith respect to the image and a subsequent image to be acquired afterthe image, and the feature point-based orientation estimation unit toestimate the position and the orientation of the image capturing unit; ashift preparation start determination unit that calculates, from a firstimage acquired while the marker-based orientation estimation unitestimates the position and the orientation of the image capturing unit,at least one failure index indicating a possibility that the detectionof the marker is failed, and determines whether or not the at least onefailure index satisfies a shift preparation start reference; and aninitial orientation setting unit that sets, when the at least onefailure index satisfies the shift preparation start reference, theinitial values of the position and the orientation of the imagecapturing unit from the position and the orientation of the imagecapturing unit estimated based on the position of the marker on theimage having been acquired in a predetermined period before the firstimage.
 2. The orientation estimation apparatus according to claim 1,wherein the shift preparation start determination unit calculates, asthe failure index, at least one out of a position of the marker on thefirst image or a traveling speed of the marker, an area of the marker onthe image, an angle made between a normal line direction of the markerand an optical axis direction of the image capturing unit, andreliability of the detection of the marker.
 3. The orientationestimation apparatus according to claim 1, wherein the initialorientation setting unit sets, when the at least one failure indexsatisfies the shift preparation start reference, a position and anorientation of the image capturing unit that are estimated by applying aprediction filter to the position and the orientation of the imagecapturing unit that are estimated based on the position of the marker onthe image acquired in the predetermined period as the initial values. 4.The orientation estimation apparatus according to claim 1, furthercomprising a marker re-detection determination unit that calculates,from a second image acquired while the feature point-based orientationestimation unit estimates the position and the orientation of the imagecapturing unit, at least one success index indicating a possibility thatthe detection of the marker is succeeded, determines whether or not theat least one success index satisfies a re-shift preparation startreference, and causes, when the re-shift preparation start reference issatisfied, the marker detection unit to detect the marker from thesecond image, wherein the shift determination unit causes, when themarker detection unit is able to detect the marker from the secondimage, the marker detection unit to detect the marker with respect to asubsequent image to be acquired after the second image, and themarker-based orientation estimation unit to estimate the position andthe orientation of the image capturing unit.
 5. The orientationestimation apparatus according to claim 4, wherein the markerre-detection determination unit sets a search range that includes themarker on the second image obtained by projecting the position of themarker in the real space onto the second image based on the position andthe orientation of the image capturing unit that are estimated and issmaller than the second image, and causes the marker detection unit todetect the marker within the search range.
 6. The orientation estimationapparatus according to claim 4, wherein the marker re-detectiondetermination unit calculates, as the success index, at least one out ofa position of the marker on the second image obtained by projecting theposition of the marker in the real space onto the second image based onthe position and the orientation of the image capturing unit that areestimated, an area of the marker on the second image, and the angle madebetween the normal line direction of the marker and the optical axisdirection of the image capturing unit.
 7. An orientation estimationmethod, comprising: detecting a marker from an image acquired from animage capturing unit that creates the image of a photographed range at apredetermined photographing cycle; estimating, based on a position ofthe marker on the image and a position of the marker in a real space, aposition and an orientation of the image capturing unit; detecting, whenthe marker is not detected from the image, a plurality of feature pointsfrom the image and a subsequent image to be acquired after the image;estimating the position and the orientation of the image capturing unitbased on positions of the plurality of feature points on the image,positions of the plurality of feature points in the real space, andinitial values of the position and the orientation of the imagecapturing unit; calculating, from a first image acquired while theposition and the orientation of the image capturing unit are estimatedbased on the position of the marker on the image and the position of themarker in the real space, at least one failure index indicating apossibility that the detection of the marker is failed, and determiningwhether or not the at least one failure index satisfies a shiftpreparation start reference; and setting, when the at least one failureindex satisfies the shift preparation start reference, the initialvalues of the position and the orientation of the image capturing unitfrom the position and the orientation of the image capturing unitestimated based on the position of the marker on the image having beenacquired in a predetermined period before the first image.
 8. Acomputer-readable recording medium storing an orientation estimationcomputer program for causing a computer to execute a process, theprocess comprising: detecting a marker from an image acquired from animage capturing unit that creates the image of a photographed range at apredetermined photographing cycle; estimating, based on a position ofthe marker on the image and a position of the marker in a real space, aposition and an orientation of the image capturing unit; detecting, whenthe marker is not detected from the image, a plurality of feature pointsfrom the image and a subsequent image to be acquired after the image;estimating the position and the orientation of the image capturing unitbased on positions of the plurality of feature points on the image,positions of the plurality of feature points in the real space, andinitial values of the position and the orientation of the imagecapturing unit; calculating, from a first image acquired while theposition and the orientation of the image capturing unit are estimatedbased on the position of the marker on the image and the position of themarker in the real space, at least one failure index indicating apossibility that the detection of the marker is failed, and determinewhether or not the at least one failure index satisfies a shiftpreparation start reference; and setting, when the at least one failureindex satisfies the shift preparation start reference, the initialvalues of the position and the orientation of the image capturing unitfrom the position and the orientation of the image capturing unitestimated based on the position of the marker on the image having beenacquired in a predetermined period before the first image.